I don't agree that HR always side with managers. If a manager does something that clearly breaks employment law, then HR know they would be negligent in their duty not to point the matter out to the manager.
What is generally the case is that such discussions take place behind closed doors so people in the workplace don't get to see the reality - one thing is certain, HR often have to clear up the mess left by a trail of destruction caused by inept management. It's a scary prospect that management who have responsibility for large numbers of staff don't always have a strong command of employment law, or they know it and ride roughshod over it, and HR have to point that out.
OP what you can put money on is that any decision made, either to award you the 'entitlement ' or not, will depend on cost/benefit- do they want to keep you, motivate you, retain your services as an employee. they could say to themselves that they'll take the risk, say no and see if you take things further or put up/shut up. Conversely, they decide not to risk the monetary "save" by not giving you "the entitlement" because the cost of losing you is way more in terms of loss of your intellectual contribution, recruitment costs etc.
The big If...... If you have a contractual entitlement, which is captured in black and white in your Contract of Employment, then it is difficult to see how HR would say one thing and Management would say another. Surely if it is that clearcut, there is no discussion to be had?