When I open a parallel pane I am not presented with a list of candidates that are appropriate for use in parallel with the current text. In the desktop app I am. Now I would be happy enough if the second pane was either a parallel or a second tab. I could for example see having it function as a text and a lexicon each in one what the desktop app would consider a zone or a tab. Or in another case have it function as a true parallel. I don't know if this fix the model or whether these thoughts are of any use but here they are.
But at present I believe it is supposed to operate as a parallel. In that case it seems that there are two issues :
1. as mentioned above the list of candidates contains every module, and in fact you can open any one of them. But where the text does not make sense as a parallel - so AF-T in parallel with ESVS it would make sense to turn off the linked icon. Now I did see this happen I think after the app opened from scratch at one point then I tried, but often the panes are linked. In such cases I guess the icon is just in the wrong state because the panes do scroll linked together because of course they cannot.
2. Normally on the desktop app when I open a parallel it opens at the verse the first pane is showing. The app doesn't do this consistently. It seems to remember where it was in a text and open there, despite the fact that the linked icon is lit. Of course a quick swipe in either pane will sync them up if they are compatible.
Open some English Tool - doesn't matter which - maybe Easton - just trying to clear out texts before the test.
Then open a text - say ESVS
Then hit the parallel button
In my case right now ASV
Now ESVS is on Gen 1:1, ASV is at Ex 13:13 and the linked icon is lit. - In your case it might not be - they might be in sync - they will after a reinstall. If they are scroll a bit so you are no longer at Gen 1:1 and panes will be in sync. Then open ESVS from the library and then reopen the parallel and now they will show the discrepancy
I just noticed though that the app appears to be remembering the position of parallel texts. So if I open a text from the library it opens at the top. But if open the same text in parallel it opens where it was last located when used in a parallel. I wonder if this is to permit restoring state in the event of app shutdown when in parallel pane mode.