This is is because PP limits the duration of one clip to 24 hours. If PP feels that the clip is long and might risk of going over 24 hours, It automatically limits the value of the speed to whatever reflects a duration less than 24 hours. And it unfortunately does that the second you attempt to type.
In your case, Typing "1", Premiere thinks that you might enter a value less than 1.25 and thus making the clip longer than 24 hours, and so it defaults to 1.25.
Luckily, there is many simple effortless workarounds until a patch is released.
1- When the value defaults to 1.25, keep typing anyway. Type "100" and then delete the other numbers (Press delete instead of backspace)
2- Unlink speed and duration (Click on the little chain that is between them). you will have to roll out the clip in the timeline afterwards.
3- Shorten your clip on the timeline, then try to apply speed/duration.
Kevin-Monahan is this a bug or normal behavior?