With the thunderbolt ethernet adapter plugged in, the MacBookPro 11,1 laptop won't suspend at all.
Without it, the laptop suspends every time.
Given that the thunderbolt ethernet adapter cannot be hotplugged, best is to leave it unused for now. A USB ethernet adapter works just fine and does not interfere with suspend or resume.
Therefore a permanent workaround has been found for this issue. The MacBookPro 11,1 can now suspend and resume flawlessly, simply by closing and opening the lid.
Update: finally figured out the pattern.
With the thunderbolt ethernet adapter plugged in, the MacBookPro 11,1 laptop won't suspend at all.
Without it, the laptop suspends every time.
Given that the thunderbolt ethernet adapter cannot be hotplugged, best is to leave it unused for now. A USB ethernet adapter works just fine and does not interfere with suspend or resume.
Therefore a permanent workaround has been found for this issue. The MacBookPro 11,1 can now suspend and resume flawlessly, simply by closing and opening the lid.
Hibernate, though, still doesn't work.