In the Adhesive アヒル, 鴨 Deficiency a somewhat large deal was made about Sheldon playing "hero" (as was reflected on in The Excelsior Aquisition). As we've seen before, such as in The 落花生, ピーナッツ Reaction, and after in The Agreement Diseccion, Sheldon will do things that he normally would not do if the societal norm/expectation is that he does, with dancing and gift buying, etc. He went to the hospital with Penny because he was supposed to. Howard's mother did not need saving which 1. eliminated a reason, in his mind, to go there, and 2. gave him time to think もっと見る about where he was going. Also, he had just gotten his germaphobia heightened によって drinking from the wrong glass, so he was operating under もっと見る paranoia in The Engagement Reaction.
Some would choose to say it's simply a plot hole, but the above explanation is what I choose to believe.