When changing the time-frame to, 1 hour (for example) in a dashboard, then clicking the play button, it reverts back to the default (usually 2 mins).
When you press play, the dashboard returns to an 'optimized' time frame that displays 120 points wide. With 1 second resolution data, that means 2 minutes. With 5 minute resolution data, that means 10 hours. The idea is that the user pans and zooms and changes the time for analysis, and then pressing play resumes the optimum view width.
One can override the optimized width by editing the dashboard. There you can set the width to 3600 seconds for example, and then 1 hour will be the width that is used when the dashboard is playing.
The image below shows where this setting is hiding.