Sharepoint 2016 OnPrem Document Library New button behavior doesn't display Office Online apps

Somehow, a poweruser has messed up their site collection.  

The farm is connected to our Office Online installation and works with other site collections.  For this poweruser, when we click on a document library New button, the only option that displays is Excel and "magically" an previously created excel document pops up in excel (without clicking the excel popup link).  

Is anyone familiar with this behavior or how to fix it?

Attempts: I created a new document library, but get the same effect.  This effect happens on subsites as well.  I create a new site collection in the same farm and it works as expected.

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Edit: I exported the site and imported it into a clean site.  The import log shows:
[1/19/2018 1:17:12 PM] [-List] [wfpub] [wfpub] [9851366e-5c8c-415d-8250-b413ddaa6742]   [-2147024809] [System.ArgumentException] [urlOfFile
Parameter name: Specified value is not supported for the urlOfFile parameter.]

I suspect that the poweruser was using a "Forms" library as if it were a Document Library.  The poweruser had not developed his site too much so we ended up destroying his site and creating it anew.  

He's satisfied for now, but I'm still curious what could have caused a user to get themselves into so much trouble.
Benjamin Landfall.NET DeveloperAsked:
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Walter CurtisSharePoint AEDCommented:
It seems that the site definition was modified, which is not always a bad thing, but this time it was. It is possible to return to the default site definition via SharePoint designer and maybe via the browser, not sure about the browser.

If this happens again maybe this will be something to check.

Good luck,,,
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Benjamin Landfall.NET DeveloperAuthor Commented:
I ended up deleting the site and starting over.  The customer didn't have much in there, so he was fine with the solution.  Thanks for your insight.
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