During the heyday of the modern Mild, a breweries Best Mild would often contain a few percent of roast malt. Although Black or Chocolate, or a combination of both, would be more common.
I'd gently suggest you're over-extrapolating from a very small minority of beers. The whole concept of a dark Best Mild was confined to a small handful of breweries, mostly around the M62 corridor (plus the odd short-lived ones from nationals like Whitbread), because they were the weird ones that had a pale mild as their "ordinary" one (and their bitter would be pale as well, so a more complicated darker beer balanced the range).
Classic mild, across time and space, is made from three ingredients. Most characteristic is around 10-15% (or more) of invert #3 or similar dark sugar, then the rest is base malt (normally pale malt but sometimes mild malt which was roughly equivalent to Vienna or a light Munich) and then usually 5-10% of unmalted adjunct. Typically that is/was flaked maize, but could be eg torrified barley/wheat for better head retention when serving through a sparkler up north. Oh, and caramel (rarely black malt) for colour. See eg :
1889 Harvey's X - 83% pale malt, 17% invert #2
https://barclayperkins.blogspot.com/2019/12/lets-brew-1889-harvey-x.html
1910 Fuller's X - 74% pale malt, 10% invert #3, 10% "pale trivert" (~invert #1?), 6% flaked maize, 0.67% caramel
https://barclayperkins.blogspot.com/2022/02/lets-brew-1910-fullers-x.html
1944 Fuller's X - 83% pale malt, 12.5% flaked barley, 2% glucose, 2% caramel (supplies of sugar were rationed for brewers during the war)
https://barclayperkins.blogspot.com/2019/07/lets-brew-wednesday-1944-fullers-x.html
1946 Tetley Mild - 59% pale malt, 15% flaked barley (but over the war that had varied between maize, rice, oats and barley), 15% "ERC/G & S" (invert #3?), 11% "Barbados" (brown sugar?)
https://barclayperkins.blogspot.com/2017/03/lets-brew-1946-tetley-mild.html
All sorts of things went into mild over the years - amber, chocolate, brown to name but a few - so you will always be able to find an example or two to "prove" that ingredient X was "used in mild", but that doesn't mean it was the norm. About the only speciality malt you can generalise about is crystal, which was fairly common (particularly in the northwest and southeast) in the early 20th century but died out in most places after WWII, in my lifetime it's only really been used in northwest milds which are noticeably sweeter and richer than eg their West Midlands counterparts.
1939 Whitbread X - 76% pale malt, 13% crystal 60L, 9% invert #3, 1.66% caramel
https://barclayperkins.blogspot.com/2019/06/lets-brew-1939-whitbread-x.html
1950 Whitbread Best Ale (effectively a Best Mild, for those wanting to generalise about such things) - 87% mild malt, 7% crystal, 4% invert #3, 1.82% caramel
https://barclayperkins.blogspot.com/2019/05/lets-brew-wednesday-1950-whitbread-best.html
1955 Wilson's Mild - 40% pale malt, 34% mild malt, 9% invert #2, 8% flaked barley, 6% crystal, 3% caramel
https://cdn.imagearchive.com/aussiehomebrewer/data/attach/96/96340-WILSON-S-XX-1955-1955.pdf
I do think people put far too much weight on the
1950s Lees Best milds when even in their own terms their recipes were all over the place before simplifying things
after 1956 (presumably as dark sugar became more available?) , even though they came out of the war with a very classic mix of pale malt, flaked barley and glucose/sugar (plus black malt for colour, presumably in part due to sugar rationing affecting the availability of caramel). I'd end with a
passing comment from Ron :
We were all on Flying Bed, a proper Dark Mild, brewed the English way. That is, not coloured with f*cking chocolate malt or roast barley.