I can create a report showing the workdays in transition status, but when I add the time dimension in order to select only recent tickets (say last three Months) the results are showing also older tickets.
Each measure is related to the Time dimension differently. For example, the measure “Issues created” shows the issue count on the timeline by issue creation date (as the measure name suggests). But measure “Workdays in transition status” shows duration in transition status by the transition date - when the issue moved from one status to another.
More details on what historical measures like transition counts and duration in transition statuses represent and how those are related to Time dimension are described here: Import issue change history
If you want to change the default behavior of measure “Workdays in transition status” and filter reports by creation date (using the Time dimension), you can create a new calculated measure (Calculated measures).
Please see those two Community posts on similar use cases and examples for the calculated measure: