I have a Samsung Galaxy S7. When I close the app, I follow this procedure:
1) close the app in the background (close it in the recent apps screen)
2) disable Bluetooth
3) enable Bluetooth
4) open the app
5) push search circle button
6) when r07 is found, select it and wait that an asterisk (*) appears
7) close the device search window by pressing the x on the top-right of the screen
(step 7 is the key!)
I couldn't get this to work for me, but having had a play around for an hour I got the following to work on my S9+
1. open the app, if it is frozen
2. click on R-07 in the bottom right ... sometimes you have to really persist and hit it numerous times
3. hopefully the R-07 will be shown with an asterisk (*) otherwise go to A.
4. in which case close the device search window by pressing the x on the top-right of the screen
A. close the app in the background (close it in the recent apps screen)
B. click on Bluetooth
C. click on Roland R-07 Midi settings
D. unpair
E. open the app
F. push search circle button
G. when r07 is found, select it and wait that an asterisk (*) appears
H. close the device search window by pressing the x on the top-right of the screen
Anyway, hopefully by trial and error you will find one of Paul's or my methods will work.