Can deleted encrypted flash drive files be recovered from internal hard drive?

No, the files were on the flash drive, not on the internal drive.

Erasing a file on an external drive does not move it to the Trash folder on the Mac. Each device has its own Trash folder, even though Finder shows just one Trash folder containing the files of all device Trash folders.

Just by chance, I recovered an old file that was deleted everywhere else, when I looked at a USB drive using Disk Utility and found it half full, but nothing showing in Finder. I opened the Trash folder and there it was, waiting to be "Put back". So, the moral is, to empty the Trash after deleting from an external drive, while it is still connected, as you have done.

Though, of course recovery software may get the items back anyway and encryption of both the internal and external drives is always a good idea to prevent this.