I tried on my iPhone with iOS 14.3, captured 25 HDRs + "Keep Original" DNGs, and these processed OK for me in my free (not subscribed) Lightroom.
Did anything on you device change recently? New iOS version? New iOS settings?
Can you explain "After sometime I export them by iTunes"
Do you mean you used File Sharing between iOS --> Mac OS? (like this:https://support.apple.com/en-us/HT210598 ) I also tried this with no problem. Even after I deleted my Lightroom app cache, the remaining unprocessed HDRs completed processing.
If you can share more steps or info for your problem, it would help. I will send you a direct message to ask for a diagnostic log.