In Studio 2021, Although "FIG." is set as an abbreviation in the segmentation rules (it's actually one of the presettings), all my English source segments are split after "FIG.". So why does this not work?

In Studio 2021, Although "FIG." is set as an abbreviation in the segmentation rules (it's actually one of the presettings), all my English source segments are split after "FIG.". So why does this not work?
is set as an abbreviation in the segmentation rules
I guess you mean is set as an abbreviation in the Language Resources.
I tool a random TM from mine, and I confirm I have 2 similar entries, fig. and Fig., but not FIG..
Coud you please confirm yu have FIG., capitalised like that and with the final period?
Yes, it's "FIG." (capitalized).
I guess you know that you need to reopen the file in Trados Studio, so the TM seetings apply to get a new and re-segmented file.
Have you got more than one TM? If so, the TM with the abbreviation FIG. must be positioned at the top.
I have several TMs.
But whenever I create a project using a particular template with that particular TM, the "FIG." abbreviation is ignored.
Is that particular TM at the top? This is important, because only resouces (segmentation rules, variables, abbreviations) of the top TM are used.
Projects created with that template have only one TM that has the "FIG." abbreviation.
Is the FIG. abbreviation in the Source language? It needs to be there, regardless of Target settings. Could you confirm it’s like that?
Yes, it's in the source language.
Last thing, and I’m running out of ideas…
You need to Re-index the TM before new settings are really applied. After changing resources, always re-index the TM (or upgrade it). Then delete the old file from Studio and add it back again, so it’s re-segmented.
I’ve tested myself, with and without re-indexing, and it works as it should.