It sounds like your SD card's write logic is damaged and it's lying to you when it shows you that the files are deleted. In this case, I suggest you try toggling the write-protect switch to tickle the circuitry that might be caching a stuck value.
- Unplug your SD card after ejecting it.
- Toggle the write-protect switch.
- Connect your SD card to your current MacBook Pro.
- Delete some files.
- Eject the SD card again.
- Reverse the write-protect switch.
- Delete files.
- Repeat the above steps a few times.
Apart from that, you can also try different USB ports in case the port you were connecting is at fault. If you still can't delete files from your SD card, update the operating system of your current MacBook Pro.
Suppose nothing works, then you can reformat your SD card on Mac via Disk Utility after backing up the important files. This way, you can erase everything on your SD card and start fresh.
For more solutions, check out the following tutorials:
How to Delete Files from Seagate/WD/Toshiba External Hard Drive on Mac?