Factorio 2.1.19 Restores Grenades And Capsule Use
Wube’s September 16 fix addresses a throwing failure in 2.1.18. The corrected build remains experimental; here is how affected players can update.
Grenades and other capsules can be thrown again in Factorio 2.1.19, according to Wube's September 16 release notes. The correction is available through the experimental branch.
The failure affected version 2.1.18
Wube acknowledged the issue in the linked 2.1.18 bug report and released 2.1.19 later that day. The newer version's changelog lists the throwing fix as its only change.

Update through the experimental branch
As of September 20, Wube still lists 2.1.19 as the experimental download.
The stable version remains 2.0.77. This hotfix does not mark the 2.1 series' promotion to stable.
For Steam, Wube directs players to the experimental option under Beta Participation in the game's settings. Standalone players can enable Experimental updates under Other settings.
If the throwing failure interrupted a session on 2.1.18, check the installed version and update to the corrected build before trying grenades or capsules again.