Are (constantly) high daytime temperatures a huge problem in flower?

I'd say the RH is dipping low. Regardless of how you approach this, your VPD has quite a range to it (especially upper range). I would be concerned firstly with where your probes are in your tent, the location matters. Is it getting direct light? Is it under airflow? For example, I don't necessarily care about what is happening to the inch or two surrounding my lights or fans, I am more so interested in the conditions at canopy level. As far as temp, Bugbee has shown plants LOVING life well above 100F (with the caveat that their root system can mitigate this temperature by means of a robust living soil web). This is, of course, a proof-of-concept/boundary value case for temps, just interesting info to add. Cannabis is incredibly resilient to temperatures, just look at the geography of naturalized cannabis. While you work on your probe and validating it is dialed in to a good location, I would take an IR temp gun or something similar and check your canopy temps - true foliage temperature at the height of the canopy. This has much more strict boundaries for good cultivation than ambient air temp.