Autofill on page load is always risky, and should only be enabled for websites that you trust (and for which your URI matching is set up to avoid unintentional autofilling of unrelated web pages — use Exact matching when possible). See a recent post of mine for more information about potential risks inherent in the autofill feature.
Manual autofill can provide a net security benefit (by not leaking credentials to the clipboard), but should be individually tested for each site on which you intend to use this function. As mentioned above, if things don’t work properly at first, there are usually work-arounds that can be implemented on a case-by-case basis.
Do let us know if any of the suggested workarounds solve your problem on Reddit.