I've done some Googling on HTC mobile hotspot and ran across a thread on an Android developer forum that seems to address this.
The information I gleaned from the forum is that is a problem that has existed on HTC Android devices since version 2.3 of the OS up to and including version 4.2 (which I think is the latest Android OS). It has not been observed on any other devices other than those produced by HTC, so it doesn't appear to be an Android issue, just an issue with the HTC enhancements to the Android OS for their devices.
One user suggested that they found a workaround which is to disable then reenable WiFi on the Android device. Another user found that the problem only happens when the Android device is allowed to go into sleep mode and disabling sleep solved the problem.