Answer
Check whether Windows' printer settings have 'Let Windows manage my default printer' enabled — when that setting is on, Windows automatically shifts the 'default' printer designation to whichever printer, including a virtual one like a PDF printer, was used most recently, overriding whatever printer you'd otherwise consider the real default. Explicitly right-click the intended printer, in either the print dialog or Windows' own printer settings, and choose 'Set as default' instead; Windows will warn that it will stop automatically managing the default printer going forward, which is exactly the behavior needed to keep a genuinely fixed default printer rather than one that keeps shifting to whatever was used last.