Hello! In both versions 9.0.11 and 9.1.0, using /pokegive or /pokegive egg shiny (& /pokegive random egg shiny) for certain shiny Pokemon will provide them in non-shiny form. This seems to occur most often with Pokemon that have noticably different gendered forms, such as Pyroar (Litleo works fine) and Meowstic (Espurr works fine). There are some exceptions, such as Pokemon like Indeedee, the Shellos line, and Heracross, that have worked fine.
As of 9.1.0, the following Pokemon have been tested/observed to return non-shiny forms: Pyroar, the Frillish line, the Shinx line, Basculegion, Combee, Meowstic, Alcremie, Vivillon, the Flabebe line, the Hippopotas line, Unfezant, and Wobbuffet.
Additionally, using /pokegive with Jellicent in general (both for shiny/non shiny) has opposite-gendered sprites and models when sent out.

Thank you for your time!

Thank you for this report! I have moved both the issue with gendered formed Pokemon not being given as shinies as well as a separate issue about Indeedee's forms to our internal tracker. Otherwise the Shellos forms you mentioned work because they are not gendered forms and are not the same kind of form as Vivillon/Alcremie/Flabebe line. Heracross is also unaffected since its gendered form has not been implemented into the mod. Otherwise, the bug with Vivillon, Alcremie, and the Flabebe line (which is unrelated to this one) has already been reported (tracker.php?p=2&t=19205).
As for the issue you mentioned with Jellicent, I was unable to reproduce this though I am not sure if I was properly understanding your description. If you test this again and it is indeed an issue in our latest version, please feel free to make a separate ticket further explaining what the issue is and how to replicate it.
Thanks again for reporting!