With sometimes massive Gaia additions to the AAVSO database, for my HADS targets I see like 5-10% of established AAVSO comp stars for example for HADS star work as being flagged VARIABLE in the Gaia DR3 database.
If it’s a long period variable, it won’t hurt HADS comp stars too much, apart from the error of the given magnitudes which become higher if the Gaia star happens to have high amplitude.
Not sure what to do with such comp stars. In some fields for a HADS target you don’t get many AAVSO comp stars, so I don’t like to exclude some of those because of the variable flag. Example: AUID 000-BPC-198 in the T CrB field is flagged as VARIABLE. Should these be reported somewhere?
As has been discussed on this forum elsewhere, a lot of the GAIA3 variables are either spurious (or variable only at the millimag level), or are some other sort of variable than they’re classified as. The example of AUID 000-BPC-198 (TYC 2037-1416-1) is claimed to be a rotational variable of 0.03 mag range in the GAIA G (red), but no period is shown in VSX. There is a good LAMOST4 spectrum showing no CaII H&K emission, which you would expect in such a star. The LAMOST and GAIA3 spectra indicate the star has a G8V spectral type.
There are other sources of photometry. An example is ASAS-SN, where the star is faint enough to have a reliable trace. The star in this case is not obviously variable, though the internal scatter is ~0.03 mag, about as good as the data are.
If you have ‘many’ nights of data that include this star (rather too faint to be a good comp for T CrB itself), try measuring it against some of the brighter comps to see if it is variable in your own data. If variable, one would expect a period of about a week, so data spanning several weeks with modest cadence should reveal whether it is variable or not.