The input method sync feature was implemented according to  issue 298345 .

Michael, can you please help to triage this? Thanks.

Steven, have you seen an existing profile on a device suddenly gain/lose input methods, apart from the first time the profile syncs on that device? If so, that's a bug, as far as I know.

We basically implement the behavior in this comment:

https://bugs.chromium.org/p/chromium/issues/detail?id=168047#c9

It's only when you sign in to a device for the first time that your synced settings are fetched, and combined with whatever input method you used to sign in.

After you have set up a device, input methods should never be added or removed by sync. (When you change input method settings, we do sync those changes upstream to Google, but only so that we know which input methods to add when the user sets up a new device.)

It is possible that I have only seen this when signing on a device for the first time, given how frequently I reimage my devices.

I haven't done extensive testing, but here are some results:
* When logging into a new device (samus UK keyboard) without making any changes I have UK, US, and Japanese input devices available.
* When adding the account to the UK samus it correctly selects the UK keyboard.
* When re-imaging the device it also correctly selects the UK keyboard.
* Neither event affected my US device

So, everything seems normal there, although arguably showing the UK/Japanese input methods on the US device is not entirely expected, but doesn't normally seem to cause problems.

I will keep an eye out for this bug and update if I catch it changing input methods again.

Steven, any updates? 

Does it make sense to close the bug and re-open it if you catch the buggy behavior again?

Sure. I saw something similar recently, but it switched to an input method I haven't used in a long time. I am wondering if I am somehow triggering a keyboard shortcut somehow?