TMP with fallback fonts doesn't show correctly, because of... layout, or size fitter?

The issue is most likely related to these fonts having very different metrics and more specifically Ascent and Descent lines as you can see in the image below.



As you can see in this image, the Ascent Line of NotoSans CJK is significantly taller than Calibri where depending on which font asset is assigned as the primary would impact the layout for a variety of reasons. Without going over all these potential reasons, the easiest solution would be to normalize the face metrics of these two very different font asset to make sure they blend in nicely with each other.

To normalize the face metrics of these two font assets simply adjust their Ascent and Descent line in the Font Asset Inspector as seen below. To visualize these metrics, add the TMP_TextInfoDebugTool.cs included in the TMP Examples & Extras to the text object and enable Show Characters.



As you can see above, I adjusted the Ascent and Descent Line of both Calibri and NotoSans CJK to match those of LiberationSans since (1) it might be used as fallback if it is still the default font asset assigned in the TMP Settings and (2) because this ends up being a happy medium between our two other font assets.

BTW: I could have chosen to match the Ascent and Descent line of Calibri to NotoSans CJK but given the Cap Height (top of tallest capital letter) of Calibri is so much lower than NotoSans CJK, you would likely end up with too much space between lines when only using Calibri.

There is no absolute right way of adjusting these metrics. In the end it comes down to adjusting these values to get a layout that looks good for your needs.

So give this a shot and let me know if adjusting these metrics allows you to get the visual / layout results you seek?

 

The cause of the strange behavior is the result of the "MessageText" object in your prefab having another text object child "MessageDate" which happens to also have a child SubMesh object as seen below.



As such, when the main text object "MessageText" looks for a sub mesh object for the fallback, it finds this sub child object which that is a child of MessageDate which is positioned relative to the MessageDate so the glyphs coming from the fallback end up offset.

SubMesh objects are no longer serialized but most likely this prefab was created with a previous version when they were still being serialized.

Deleting this TMP SubMeshUI object from the text objects in this prefab and other prefabs should resolve the issue. Below is the edited prefab without those sub mesh objects.



Testing with the changes




I will further investigate this to see if I can catch the issue automatically but you should be good to move forward without any future changes.

P.S. Since there is no way to have the Prefab system ignore certain objects when updating or creating prefabs, make sure the default text doesn't use a fallback which would force the creation of a sub object.

 

Since the main cause of the issue you ran into is related to sub text objects getting saved with Prefabs, I made a change where sub text object will now have HideFlags.DontSave set to prevent them from being saved with the Prefab as they are created as needed anyway.

This change will be in Preview 8 which should be out tomorrow if I stop making changes and don't run into any new issue.