YES, Anytime you experience numbness (especially as mentioned), there is a concern. Numbness is not considered acceptable, IMHO.
The only exception comes from extraordinary circumstances where you increase distance/time/etc. well past what you typically do. Otherwise, that should not happen.
You mention the harder efforts as the time that occurs. One common reaction we do when hitting hard efforts is to hinge at the hips, bend our arms and get low (or move from the hoods to the drops), to ‘dig’ for the effort.
This often leads to more anterior pelvic tilt, and therefore risk of increase pressure on the soft tissue and restricting blood flow to the related areas. This can lead to numbness.
Additionally, we can sometimes creep forward on the saddle in the same consequence as pushing harder.
This leads to our sit bones moving forward on off the proper width part of the saddle. That means we “sink” a bit and often increase pressure on the soft tissue and restricting blood flow to the related areas. This can lead to numbness (same as above).
Those are the two most likely causes I can think of considering you are fine in the lower effort cases.
I had the same thing and swapped my saddle recently and it’s a night and day difference. No idea why I was riding a saddle without a divot for 20 years. The new saddle is really flat and narrow with a cutout. I got this one PRO Homepage | Pro Bikegear . Love it. They have a pretty good saddle selector too.
I ordered the edge also. Seems to work well and I agree, great company. Shane is very responsive to all questions and commited to getting you dialed in.
Maybe. There are no absolute answers here, because we are working with a human body. Each one is a bit different and there is no guarantee that a solution for one will be a solution for all.
What I can say is that I and many others still have comfort issues inside in a rigid trainer, even when we are well fitted on a good saddle and have no issues outside. My theory is the static nature of rigid trainers leads to unnatural loading on the sit bones and soft tissue. That realization is what lead me down the road of making, using and recommending rocker plates.
more info about them here if you want to see what they are and how they work.
Even with all the tricks and proper setup, I still stand regularly when inside. I do it at least as often as I do when riding outside, if not more. The breaks are still helpful since there are still minor differences compared to outside.
this thread has a broader range of discussion and options for pain and numbness
Odd your fitter says everything is good. IMO your fit is off. You just move around enough outside to not notice.
To your question - get this fixed. Do not try to tough it out. A saddle isn’t a bad place to start - but cost comparison - id look at another fitter as well