Legacy ibus-pinyin release you currently shipping is designed to work with IBus 1.4.x. As you've updated the IBus stack to IBus 1.5.x, you need to make sure the engines are also recent enough.
Legacy ibus-pinyin release also has many known bugs that have zero chance of getting fix.
Do not underestimate the importance of this bug.
Legacy ibus-pinyin release you currently shipping is designed to work with IBus 1.4.x. As you've updated the IBus stack to IBus 1.5.x, you need to make sure the engines are also recent enough.
Legacy ibus-pinyin release also has many known bugs that have zero chance of getting fix.