bug(linux): wrong context after backspace in Firefox with Wayland

Start `ibus-engine-keyman` from the command line so that you can see the output:

- make sure a non-Keyman keyboard is active
- stop ibus with `ibus exit`
- start Keyman with `/usr/libexec/ibus-engine-keyman`
- in a second terminal, start ibus with `ibus start`
- when switching to a Keyman keyboard you should now see output for each keystroke in the first terminal

In Firefox when running Wayland open [keymanweb.com](https://keymanweb.com) and type
```
abc
d
```
with a Keyman keyboard. Then press Backspace.

**Expected context after backspace:** either `abc\n` or no context
**Actual context:** `d`

While this is not visible to the user and things usually recover on the next keystroke, this can lead to the context getting out of sync when typing fast. Might be the reason for #15550 .