plotting – Can’t use ReplaceAll in Plot

I want to plot a custom function with a label showing my defined symbol for that function.
I have had success in using Plot for plotting my function directly, however, now that I’m trying a somewhat more complex combination of my function it no longer works.

I’ve tested my function and the replacement rule using Table and the values it throws are correct. Also, the replacement goes through flawlessly on the label.

While I do want a solution to my issue in general, I would greatly appreciate an explanation on why my snippet doesnt work as intended.

The expression in question is:

Plot(f(u, t), {t, 0, 6}, PlotLabel -> f(HoldForm@u, t)) /. 
 f -> (5 #1(#2) - 11 #1(#2 - 1) + 6 #1(#2 - 2) + 8 #1(#2 - 3) &)

The resulting graph looks like this:

Failed plot

The graph, however, comes out well using With, but I feel it looks a bit more confusing.

With({f = (5 #1(#2) - 11 #1(#2 - 1) + 6 #1(#2 - 2) + 8 #1(#2 - 3) &)},
  Plot(f(u, t), {t, 0, 6}, PlotLabel -> f(HoldForm@u, t)))

Intended graph

I’ll have to go this route in the meantime, but I don’t particularrly like that solution.

PS: I know that there’s a function called UnitStep that would simplify things greatly, however I was required to define my own function. And I know solving my issue would be useful in the future.