I'm having exactly the same issue as the original poster..........so glad I'm not the only one.
This is very frustrating as I use the task switcher (alt + tab) all of the time. I too had noted that for 'internal emails' it was not having an issue, but for 'external emails' it was happening - and I would say atleast 75% of the time it was happening.
In regard to the release of the Tab or the Alt key first......I've found that you have to release the Tab key first anyway for the switcher to work.......so was unable to recreate the use of releasing the Alt key first.
This issue was definately not happening in prior versions of Lotus Notes - we've been on notes since around 2005 or prior and I've never had this issue before.
I hope IBM can come up with a solution to this soon as it's very very frustrating