I can't replicate this.
When I format a cell as MMM-YYYY, and type in exactly this
1/4
it displays as
Apr-2019
How are you entering the 1/4 value ?
By typing it in ?
Or pasting it from somewhere else ?
Or maybe by macro ?
O
Were the cells formatted as Text before you changed the format to MMM-YYYY? and is the 1/4 aligning to the left or the right?
Hmm, the default format should be General on a newly opened sheet with no data.