I think it's fine/tolerated, a simplification from the full/stricter:
It was only after I closed the door that I remembered (that) I had forgotten the keys.
The final relative ("that") in this one above allows a tense simplification in the
subordinate after it (
I had forgotten->I forgot, past perfect-> simple perfect, see Swan) after it and it may be
eliminated itself without loss of clarity.
Also, "It's" is used many times in this "atemporal" way, for past
situations, but mostly in informal writing, but also in the New
York Times:
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... radio market. It's only after Mr. Stern turned against Mr. ...
March 21, 2004
- By FRANK RICH (NYT) - Arts
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However, you may want to consider other, stricter, alternatives:
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It was only after I closed the book that it dawned on me that feeling
better in this way made everything worse.
By JUDITH SHULEVITZ,
The New York Times
http://tinyurl.com/ym7qyf
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