I should have mentioned that I've filed feedback. I don't think an actual bug report wouldn't be appropriate in this instance. I have installed the recent "macOS Mojave Supplemental Update, Safari 14 with bug fixes" and tried again at the site where the Captcha was reliably not functioning. It now works.
ryck
"What Were Once Vices Are Now Habits" The Doobie Brothers
iMac (Retina 5K, 27", 2020), 3.8 GHz 8 Core Intel Core i7, 8GB RAM, 2667 MHz DDR4
OS Sonoma 14.4.1
Canon Pixma TR 8520 Printer
Epson Perfection V500 Photo Scanner c/w VueScan software
TM on 1TB LaCie USB-C
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