The only place anyone will get a specific data sheet is from the actual manufacturer of the part in question. You must ask them for it. If they do not have one them perhaps you can select a higher quality lamp from a company that has and will supply a data sheet.

This is supposed to be a test rig so you need to be able to replace the bulb and be confident that the rig will work as designed. you can only do that if you know that your design will work correctly with bulbs that exibit the full range of data sheet values.

The other option is to buy a box of bulbs ans hand pick them to be certain that they fall within your design parameters. With this option you need to specify how to select morre bulbs for that day down the road when the last preslected bulb fails.

The situation that you describe is covered under the second possible case. With this bulb as a load the actual current flow will be less than the constant current source is intended to provide because the lamp is limiting the available voltage to the source to below that needed for it to operate properly.
Had you looked at the data sheet (were it available to you) you would conclude that this lamp is not suitable in this design. to correct the problem you could either raise the input voltage to ensure that the worst case bulb will leave sufficient voltage available for the constant current source to work correctly or select a different bulb that needs less voltage across it at 200 ma.
Your choice but to make it you do need to have correct information on the components that you chose to work with.
Terry