Hi Markus,
It is interesting that you've found the XS battery to be functional in the X. There are several factors that come into play when Apple is designing a battery for a given phone, but it seems that the changes between the X and the XS weren't significant enough to keep the batteries from being compatible. It's not at all uncommon for the connector itself to change, the position on the motherboard, the physical dimensions and the sensors contained in the BMS to be different.
Anyway, assuming it worked at all, there's absolutely no reason to think it won't keep working. If the connector was different it wouldn't have plugged in, and if the position had been moved the connector wouldn't have reached. Obviously it physically fits inside the case, so the dimensions are close enough. Finally, I assume you're not seeing any weird battery health readings or experiencing any reboot loops, which would say that the internal sensors are working too.
It appears the XS battery has a slightly smaller capacity, but that won't affect the normal operation of the phone outside of needing to be recharged a few minutes sooner than it would with the original X battery.
So yeah, if you've been using it for a few days and aren't seeing problems, then you should be good; there's no reason it shouldn't just keep on working.
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