Unexpected `:extend()` behavior (with `selector { selector {} }` nesting)

My Input

So this:

.class, .other { /* stuff */ }
.class {
    .class, .other { /* more stuff */ }
}
.extension { &:extend(.class all); }

Current (1.4.1) Output

Currently compiles to this:

.class,
.other,
.extension {
    /* stuff */
}
.class .class,
.class .other,
.extension .extension,
.extension .other {
    /* more stuff */
}

What I'd Expect

But it seems like the result should be this:

.class,
.other,
.extension {
    /* stuff */
}
.class .class,
.class .other,
.class .extension, /* this */
.extension .class, /* this */
.extension .other,
.extension .extension {
    /* more stuff */
}

In other words, (almost) exactly like this compiles:

.class, .extension, .other { /* stuff */ }
.class, .extension {
    .class, .other, .extension { /* more stuff */ }
    /* OR */
    & &, .other { /* more stuff */ }
}

Questions

So,

• Is there a reason for the current behavior vs. the other?
• What's the recommended way to get the target output? (for my best effort, see "Workaround" below)

Reproduce the nested selector and :extend() examples with Less 1.4.1, then compare the current and expected CSS outputs shown in the issue. Determine whether the expected selector expansion is intended; done means the behavior is explained and, if changed, the compiler produces the requested output without regressing the demonstrated cases.