It’s well-known that flash storage and SSD have inherent problems.
The research indicates that the secure delete mechanism is implemented incorrectly in 90% of such units. Even where it is implemented correctly, data is often still recoverable.
The only option is encryption. If you’re using AxCrypt then make sure you only ever copy encrypted .AXX files to the device; never copy plaintext.
Ideally you want full disk encryption [BitLocker]. Then the data is unrecoverable without the password unless somebody captures the device whilst unlocked.
Even with full disk encryption you still need file level encryption [AxCrypt] if you intend sharing the files with friends/colleagues or if you want to upload them to your cloud storage.
If you don’t want to use full disk encryption, or you’re worried that your sensitive unencrypted data can be recovered, then the only safe option is to physically burn the device or destroy it in an industrial shredder.