Can you post a screenshot of what the NXP TagInfo app says when you successfully scan your tag (rammed right up to your phone). Here’s my xNT being scanned and you’re right, it says “Strong load modulation enabled”:
It will give us more info about your tag and possible insights into why your load modulation would be set to “normal” (not the default).
Here is a diagram I found of the NFC antenna on your phone:
The M8 has an antenna that is very poorly shaped for coupling with the helical antennas in the x-series tags. It’s also traveling through a metal casing.
Since you seem pretty familiar with sending commands to the tags, you could try using the information in page 19 of this document [PDF] to set the Strong load modulation to enabled. If your implant is an xNT, I recommend you purchase a few NTAG216 stickers online and practice with them. You may also want a better NFC reader to interact with the tags during testing.