I have also checked if this was any different in the past and checked Bionic (thanks Frank for the system). It behaved the same way (i.e. no regression).

Unmapping device:

Jan 25 05:07:15 hwe0006 kernel: sd 1:0:0:1074151462: [sdc] tag#1877 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
Jan 25 05:07:15 hwe0006 kernel: sd 1:0:0:1074151462: [sdc] tag#1877 Sense Key : Aborted Command [current] 
Jan 25 05:07:15 hwe0006 kernel: sd 1:0:0:1074151462: [sdc] tag#1877 Add. Sense: Logical unit not supported
Jan 25 05:07:15 hwe0006 kernel: sd 1:0:0:1074151462: [sdc] tag#1877 CDB: Write(10) 2a 00 01 04 88 00 00 00 08 00
Jan 25 05:07:15 hwe0006 kernel: print_req_error: I/O error, dev sdc, sector 17074176
Jan 25 05:07:15 hwe0006 kernel: device-mapper: multipath: Failing path 8:32.
Jan 25 05:07:15 hwe0006 kernel: sd 0:0:0:1074151462: [sda] tag#2231 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
Jan 25 05:07:15 hwe0006 kernel: sd 0:0:0:1074151462: [sda] tag#2231 Sense Key : Aborted Command [current] 
Jan 25 05:07:15 hwe0006 kernel: sd 0:0:0:1074151462: [sda] tag#2231 Add. Sense: Logical unit not supported
Jan 25 05:07:15 hwe0006 kernel: sd 0:0:0:1074151462: [sda] tag#2231 CDB: Write(10) 2a 00 01 04 88 00 00 00 08 00
Jan 25 05:07:15 hwe0006 kernel: print_req_error: I/O error, dev sda, sector 17074176
Jan 25 05:07:15 hwe0006 kernel: device-mapper: multipath: Failing path 8:0.
Jan 25 05:07:15 hwe0006 kernel: sd 1:0:1:1074151462: [sdd] tag#1877 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
Jan 25 05:07:15 hwe0006 kernel: sd 1:0:1:1074151462: [sdd] tag#1877 Sense Key : Aborted Command [current] 
Jan 25 05:07:15 hwe0006 kernel: sd 1:0:1:1074151462: [sdd] tag#1877 Add. Sense: Logical unit not supported
Jan 25 05:07:15 hwe0006 kernel: sd 1:0:1:1074151462: [sdd] tag#1877 CDB: Write(10) 2a 00 01 04 88 00 00 00 08 00
Jan 25 05:07:15 hwe0006 kernel: print_req_error: I/O error, dev sdd, sector 17074176
Jan 25 05:07:15 hwe0006 kernel: device-mapper: multipath: Failing path 8:48.
Jan 25 05:07:15 hwe0006 kernel: sd 0:0:1:1074151462: [sdb] tag#2231 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
Jan 25 05:07:15 hwe0006 kernel: sd 0:0:1:1074151462: [sdb] tag#2231 Sense Key : Aborted Command [current] 
Jan 25 05:07:15 hwe0006 kernel: sd 0:0:1:1074151462: [sdb] tag#2231 Add. Sense: Logical unit not supported
Jan 25 05:07:15 hwe0006 kernel: sd 0:0:1:1074151462: [sdb] tag#2231 CDB: Write(10) 2a 00 01 04 88 00 00 00 08 00
Jan 25 05:07:15 hwe0006 kernel: print_req_error: I/O error, dev sdb, sector 17074176
Jan 25 05:07:15 hwe0006 kernel: device-mapper: multipath: Failing path 8:16.
Jan 25 05:07:15 hwe0006 kernel: sd 1:0:0:1074151462: Power-on or device reset occurred
Jan 25 05:07:15 hwe0006 kernel: sd 1:0:0:1074151462: alua: port group 00 state A preferred supports tolusnA
Jan 25 05:07:15 hwe0006 kernel: sd 0:0:0:1074151462: Power-on or device reset occurred
Jan 25 05:07:15 hwe0006 kernel: sd 0:0:1:1074151462: Power-on or device reset occurred
Jan 25 05:07:15 hwe0006 kernel: device-mapper: multipath: Reinstating path 8:32.
Jan 25 05:07:15 hwe0006 kernel: sd 1:0:1:1074151462: Power-on or device reset occurred
Jan 25 05:07:15 hwe0006 multipathd[591]: sdc: mark as failed
Jan 25 05:07:15 hwe0006 multipathd[591]: mpatha: remaining active paths: 3
Jan 25 05:07:15 hwe0006 multipathd[591]: sda: mark as failed
Jan 25 05:07:15 hwe0006 multipathd[591]: mpatha: remaining active paths: 2
Jan 25 05:07:15 hwe0006 multipathd[591]: sdb: mark as failed
Jan 25 05:07:15 hwe0006 multipathd[591]: mpatha: remaining active paths: 1
Jan 25 05:07:15 hwe0006 multipathd[591]: sdd: mark as failed
Jan 25 05:07:15 hwe0006 multipathd[591]: mpatha: remaining active paths: 0
Jan 25 05:07:15 hwe0006 multipathd[591]: 8:32: reinstated
Jan 25 05:07:15 hwe0006 multipathd[591]: mpatha: remaining active paths: 1


mpatha (36005076306ffd6b60000000000002606) dm-0 IBM,2107900
size=64G features='3 queue_if_no_path queue_mode mq' hwhandler='0' wp=rw
`-+- policy='service-time 0' prio=0 status=active
  |- 0:0:0:1074151462 sda 8:0  active faulty running
  |- 0:0:1:1074151462 sdb 8:16 active faulty running
  |- 1:0:0:1074151462 sdc 8:32 active faulty running
  `- 1:0:1:1074151462 sdd 8:48 active faulty running


Mapping device back:

Jan 25 05:07:16 hwe0006 multipathd[591]: 8:0: reinstated
Jan 25 05:07:16 hwe0006 multipathd[591]: mpatha: remaining active paths: 2
Jan 25 05:07:16 hwe0006 kernel: device-mapper: multipath: Reinstating path 8:0.
Jan 25 05:07:16 hwe0006 multipathd[591]: 8:16: reinstated
Jan 25 05:07:16 hwe0006 multipathd[591]: mpatha: remaining active paths: 3
Jan 25 05:07:16 hwe0006 kernel: device-mapper: multipath: Reinstating path 8:16.
Jan 25 05:07:16 hwe0006 multipathd[591]: 8:48: reinstated
Jan 25 05:07:16 hwe0006 multipathd[591]: mpatha: remaining active paths: 4
Jan 25 05:07:16 hwe0006 kernel: device-mapper: multipath: Reinstating path 8:48.
Jan 25 05:07:16 hwe0006 multipath[2853]: dm-0: usable paths found
Jan 25 05:07:16 hwe0006 multipath[2860]: dm-0: usable paths found
Jan 25 05:07:16 hwe0006 multipath[2867]: dm-0: usable paths found


So this isn't a behavioral regression either.
Waiting for your feedback on your use case and expectations.