Glory was technically the bigger flop but Joanne's underperformance is more detrimental in my opinion. Britney has been in the business for almost 20 years and its not exactly shocking that this album didn't do super well as her whole brand was based off of sex appeal which she has now been replaced by younger and hotter popstars.
Gaga is only 8 years into her career and completely switched sounds and alienated most of her core fan base. She probably hoped to gain a more mature, adult oriented one, but a lot of that demographic still sees her as a joke because of all the antics she used to pull back in her prime. Not to mention all the extensive promotion for Million Reasons and it's still not really taking off... and Perfect Illusion completely tanking... she needs to play her cards right and release a more "Gaga" oriented album by next year or she's gonna lose most public interest tbh :/