PadKeys and Simplenote
I've noticed the following problem using the PadKeys 3rd-party keyboard with the iOS Simplenote app (iPad 6, iOS 12.4.1, PadKeys 4.5.1, Simplenote 4.9). Figure 1 shows the setup (I shrank these screenshots using Pixlr, to save bytes). Each line of text ends with a new line. I select one line of text by double tapping and dragging the handles, then select Copy from the popup menu. I then position the cursor by tapping at the end of another line (Figure 2) and hit Return (Figure 3) in preparation for pasting the copied line. But this action erases the last character of the previous line, which is inconvenient. It is as if the new line entry is replacing the last character on the line.
This does not happen if I use the Apple keyboard with Simplenote, nor does it happen with PadKeys and Apple Notes. Also, it does not happen if I use the PadKeys cursor move keys (<>) before entering the Return. This is a viable work-around. And it does not happen if I create the blank line (hitting Return) prior to the Select/Copy operation. But it does reveal a glitch in the communication between PadKeys and the Simplenote app. Is there some confusion regarding the line end marking (cr, cr/lf, lf) used by the two apps?
It looks like the screenshots appear in reverse order. Sorry for the confusion.

Thank you for the clear report. The problem seems to be due to a bug in SimpleNote.
Once you’ve selected the text, copied, and tapped the cursor to where you want to insert it, there is no longer any selected text, however SimpleNote tells PadKeys that there is. This cause PadKeys to execute a ‘Delete’ action, since ordinarily when you hit ‘Return’ with text selected, that’s what should happen. Other apps do not do this, and presumably Apple’s own keyboard has other ways of detecting that there IS actually no selected text to delete.
If possible, please report the bug to SimpleNote. (We’ll try to report it as well.)
In the meantime, a work-around is the move the cursor right and left after positioning it. This jogs SimpleNote to sync the selection state to the system.