Is the Sonim XP3plus the only phone with FM radio available from T-Mobile right now?

Just because it has a "chip" doesn't mean the coms are set up.  This has been a peeves of mine(not directed at you) trying to explain just because it has a Qualcomm chip that supports FM for example doesn't mean the rest of the hardware is connected to make it work.  If it has no 3MM headphone jack for example it doesn't have antennas for it.  Even if it has a 3MM jack it may not have the chip comm connected. Also Qualcomm has to be paid for licensing and FCC has to test it and approve it.  If you look at the FCC code given and you don't see FM radio testing = It shouldn't work and if it does then Samsung(or any manufacturer) has to disable it.  So before you buy MAKE sure you check the FCC filings, and also because it's in 1 country with it doesn't mean it's the same device.  It may have the same name, but the logic boards may be different in parts numbers.  Also buying from overseas doesn't mean it will have all the same bands you need to have service in the states for example.  Carriers require VoLTE with all their bands, but those devices might not have those bands or only a fraction of them.

For example the Nexus 4 could do LTE, but Google removed everything needed and blocked it from future updates or else they’d over Qualcomm money and also FCC can fine Google.
https://www.anandtech.com/show/6745/nexus-4-jdq39-422-ota-update-removes-unofficial-lte-on-band-4

Also keep in mind US is planning on removing FM in the near future.  They've been talking about it since early 2Ks.  There has been push back but at some point they want to free up airwaves for other things, and want FM to go digital.  Some FM stations already have analog and digital versions.  AM wouldn't be a surprise to be gone in the very near future as AM has been missing in a few cars out right now, and more will follow suite for sure.