Having converted an entire site to also display on a smartphone, I found that one specific form disappeared from the desktop version in multiple browsers.
Each time the screen loaded on desktop, the form appeared and then vanished. The mobile variant was OK.
Other forms slotted into/replacing the problem form on the same viewed OK.
When I deleted the problem form from the mobile variant, the desktop version was OK.
Have now deleted all pages of the mobile variant version until I can be sure the problem will not reoccur.