'Saving as type' randomly resets to pick PNG, while I always export PDFs

Just in case: You can directly choose the desired PDF preset from the File > Adobe PDF Presets > … menu (as you may know anyway). 

The disadvantage of this approach is that InDesign doesn’t provide a direct way to assign a keyboard shortcut to the desired preset (as far as I know). This can be achieved with a 3rd-party macro utility or by creating a script to call the desired preset (to which you can assign a shortcut in InDesign).

Hi ​@CZBrand_L*SH,

Thanks for reaching out, and sorry to hear about the frustration.

I'd like to gather a few more details so I can better understand what's happening. Could you please confirm your exact InDesign version and operating system version? Also, are you seeing this behavior when using Export or Save As? I'm asking because the Save As dialog in InDesign only supports INDD, INDT, and IDML formats, so it wouldn't normally default to PNG.

Could you also let me know if this happens with all documents, including a brand-new document, or only with specific files? If you haven't already, please try resetting InDesign preferences by following the video at the bottom of this page and let us know whether that changes the behavior:
https://helpx.adobe.com/indesign/using/setting-preferences.html

If possible, please share a short screen recording showing the workflow and the format switching back to PNG. 

Looking forward to your update.

Abhishek