You might be onto something here. I saw with wifi scanning that iPhones try to choose an unused or “best” channel and they might end up with higher channels. If those aren’t available to your Beryl then it won’t search these channels and thus connect. Besides using 2,4, you might want to turn off all nearby 5 GHz AP (or use a makeshift Farady cage) before enabling personal hotspot. Iphone might then choose a lower channel that your Beryl is ok with.
Well Beryl only shows 128 max, even though I have another EU country set. 
Channels >= 149 have seriously reduced transmit power in Europe so maybe they consider these useless. When I switch country to US, these channels become available, so it’s definitely country related.