This is strange, as the release of the right-shift key triggers the binding for the left-shift release. This issue, however, is not a tkinter issue (which just passes the calls onto Tcl/Tk), as running the following in 'wish' prints "R shift down <newline> L shift up":

bind . <Shift_L> {puts {L shift down}}
bind . <KeyRelease-Shift_L> {puts {L shift up}}
bind . <Shift_R> {puts {R shift down}}
bind . <KeyRelease-Shift_R> {puts {R shift up}}

You can take this issue up with the Tcl team if you want (https://core.tcl-lang.org/index.html), however, I ask that you close this issue as third party.