Yes, such an LPG or CNG vessel is in the scope of the PED and must be assessed according to its maximum allowable pressure and volume.
Reason:
An engine powered fork lift truck is not a motor vehicle in the sense of Directive 2007/46/EC*), so the exclusion of the Article 1 paragraph 2(e) does not apply.
Note 1:
Transportable gas cylinders which can also be used for fork lift trucks are in the scope of ADR and as such are excluded from the PED, due to Article 1 paragraph 2(s).
Note 2:
The same applies to similar machinery not covered by Directive 2007/46/EC*).
Remarks by PED ONLINE Team:
*) The Directive 2007/46/EC has been replaced since 1 Sep. 2020 by Regulation (EU) 2018/858