The journal "Research in the Mathematical Sciences" was founded in 2014 and originally published by SpringerOpen, a division of Springer supporting Open Access journals. In the first article of the introductory issue (https://link.springer.com/article/10.1186/2197-9847-1-1) Ken Ono emphasizes the journal's commitment to Open Access publishing (albeit with a funding structure based on authors paying article-processing fees).
But if you go to the old website (https://resmathsci.springeropen.com/) of the journal, you are now greeted with this message:
As of January 1, 2018, Research in the Mathematical Sciences (RMS) has transitioned from an Open Access journal to a subscription-based journal with a hybrid Open Access option. RMS continues to be published by Springer Nature and an archive of all articles previously published in the journal is hosted here. All submissions going forward will be considered for the subscription-based journal. Please see the RMS website for more information.
I find it very surprising that within 4 years of existence this math journal had to change its fundamental publishing philosophy.
Is there any more information, beyond the above quote, about what happened to "Research in the Mathematical Sciences" and why it abandoned its original Open Access vision?
EDIT: As mentioned in the comments, essentially the same thing happened with "Research in Number Theory" (see https://resnumtheor.springeropen.com/), and Ken Ono's response also addresses this journal as well.