Yes, the Bolians should be discovered in between TOS and TNG and left to sequel shows. But its not hard canon. At no point did TNG/DS9/VOY ever say Starfleet did not meet the Bolians or have Bolian crewmembers during the TOS era. Just like virtually no Andorians in those shows didn’t mean that the Andoria left the Federation.
And why shouldn’t it be asked if aliens never seen in the series prior to ENT or the Kelvinverse would break the immersion were they to appear in a TOS environment? Particularly for you, who wants the show to reflect TOS as it was in the ‘60s, Instead of how they are remaking and updating the TOS era for today? Since ENT tried to reflect TOS to a degree while introducing new aliens but lost viewers, DIS tried to reflect TOS to a degree while visually updating everything but gained intense criticism, and the Kelvinverse offered its take on TOS in what was explicitly an alternate universe, and some fans just did not take to Abrams’ vision. Nothing is coming close to 100% accuracy of TOS unless its animated.