One possible cause that has afflicted others: Unbelievable as it sounds, many camera manufacturers still don't record fractional seconds for capture time in photos' metadata, and LR often doesn't handle that situation very well, depending on several factors. There are different possible workarounds, but let's first determine if that's what's going on for you. Upload two sample pics that don't sort correctly to Dropbox (or similar) and post the sharing links here, and I can put them under the microscope.
Without seeing the images in question, most likely this is a problem with the sub-second parsing when doing bursts of images.
I solve this problem, and the multiple camera problem, by naming my files as yyyymmdd_hhmmss_nnnn where nnnn is the original file number. The timestamp part of the name gets me the multiple camera sync since I will not be using both cameras in the same second. The original file number gets me the correct sub-second sorting. With this format, I just sort by filename.
When I prepare my cameras for the next event, while I am formatting the cards in the camera, I manually reset the file number back to one.
problem with the sub-second parsing
I've investigated a number of instances of these symptoms, and in all of them, the cause was that the camera wasn't recording fractional seconds in the industry-standard field. But it would be good to verify that's what is going on here.