I couldn't say why exactly ... I have so many blockers/obfuscators/spoofers in my browser that it could be any one of them and I'm not prepared to troubleshoot it just to accommodate the Medium devs' everchanging codebase ¹ (I'd sooner just stop using the site) ... but, since January (I think), I haven't been able to paste anything into a post/reply and have to insert it from the action menu. So, if it doesn't fall within the very limited range of actions supplied, I can't do it any more.

Medium was never good as such and has become increasingly limited in the meantime, but these days it's downright mediocre — the only thing it offers is reach (and then really only the in US ²).

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¹ I have to turn off CleanURLs because it prevents me from listing people or tags, but that's all I'm prepared to disable when using Medium.

² It's not FB, Insta or the Tok and will likely fall under the Where Are They Now? (MySpace and Other Failures of Yesteryear) rubrik sooner rather than later.