Does this repro in M53?  And could I see a video of the symptoms?

This is reported from play store comment with 34 other users like this post in 1 day. We're still trying to find Xperia phone to reproduce this issue. 

We don't see any keyboard issues on an Xperia Z5 (E6653).

Do the reports include any example sites or exact steps?

Hmm, OK, if there's 35 user reports, there's likely a real issue here.  But we couldn't repro locally on an Xperia Z5.  Maybe it only affects certain particular versions of Xperia keyboard.  I can take a look as soon as we find a repro device.

Is there any way I can search the Play store comments myself to mine them for details?

There's an archive of old Xperia keyboard versions at http://www.apkmirror.com/apk/sony-mobile-communications/xperia-keyboard/ .  They refuse to install on non-Sony devices, though (need shared library com.sony.device).

Googling reports that newer versions of Xperia keyboard removed T9 (numpad-style) keyboard support.  So this would only affect older ones.

You can search M54 play store comments here: https://hotsauce.corp.google.com/product/282/neutron?lView=td&lTagFamily=%23issue&lTagSearch=keyboard&lTagId=a11:69647b14:930357&lRSort=1&lROrder=2&lRFilter=1&lCategory=8590027391&lReportSearch=tag:%23label%3Eclankm54

Is there anything I should get from this user to help us investigate? 

Thanks

The most useful information would be keyboard version from Settings -> Apps -> Xperia Keyboard.

You claimed 35 reports, but I only see one report for searching for "Xperia"?  Where does the number 35 come from?

I installed the oldest available version of Xperia keyboard, 6.4.A.1.24, and configured it in phonepad mode (settings -> Languages and Input -> Xperia Keyboard settings icon -> Full Keyboard/Phonepad), but still cannot repro a problem.

So far, the indications I've seen are that this issue is highly niche and not worth blocking 54 on.

Closing as cannot repro.