I've had some struggles with tables seemingly spontaneously reformatting themselves while working in Microsoft Word through MS Teams. If, for example, I manipulate rows and columns, split cells, merge cells, etc., until my table looks as desired, other members of my team, upon accessing the file, might see something quite corrupt, relative to what I'm looking at. For an example of the discrepancy in appearance of the two tables, please see the attached images. Note that I have placed opaque rectangles over the data in my colleague's screenshot, and replaced all the content in my screenshot with x's, because the information was sensitive - but please know that this is not the reason for the incorrect formatting. The formatting was incorrect even when the content of the two tables was identical.
Both of us are in the same document, looking at the same table, simultaneously. My first thought was that this table style is based on "Normal", and perhaps my teammate had "Automatically Update Document Styles" checked in their "Templates and Add-ins" settings, but I confirmed that that is not the case for either of us. If it were checked, I suppose MS Word might look for the "Normal" template on the user's local machine, attach that template, override the "Normal" table style, which would then propagate through to this custom table style.
I've been having a similar struggle in that table cell shading sometimes gets removed. For example, that light blue shading is not part of the custom table style. Occasionally, upon opening the document, I will find that this shading is gone. After reapplying it several times, it seems to stick. But I can find no rhyme or reason as to why MS Word seems to be spontaneously reformatting my tables.