@phm2000 I don’t know that model but I did find that your switch does support “green ethernet” mode. I’m not saying that this IS the problem but if your only solution is to disable STP then I would look to see if green ethernet is enalbed on your switch. According to this article its disabled by default: https://community.hpe.com/t5/Comware-Based/How-to-disable-IEEE-802-3az-Energy-Efficient-Ethernet-feature-on/td-p/6896946# Look at the accepted solution.
Disabling STP on user facing ports is not the best choice. You may need to get with a switch engineer to understand why turning on RSTP does not solve your problem (because turning off STP solves the net booting). From your description it sounds like the port is in standard STP mode even if you set it to RSTP. I assume (with RSTP on) placing a dumb (unmanged) cheap switch between the building switch and the pxe booting computer will also mask the problem. We will typically recommend this for people to test to see if the problem is spanning tree or not without changing the switch settings.