Is a character with a Fixed Attitude throughline always Steadfast?

Short answer: No.

EXPLANATION:

This is a common problem when looking at any choices for a story point, and that problem is forgetting the story point itself. Fixed Attitude is a structural item in the Dramatica story structure. It's inherent meaning is fixation, bias, preconception, etc. So, taken alone it will always mean those things.

However, you also have designated that it is the Throughline you are interested in, which limits that character to the Main Character or the Impact Character. This throughline gives the context within which we examine the MC or IC at its largest structural story point -- the Class or Domain.

Your question goes on further to say that your character is "relentlessly logical." So, how does being relentlessly logical relate to Fixed Attitude?

Since Fixed Attitude has more to do with WHAT someone thinks than HOW someone thinks, let's examine "relentlessly logical" in that context. How might that be 'encoded' into a fixed attitude? How about these examples:

• Believing logic is the only valuable way to settle problems
• Being fixated on one particular flaw in logic
• Believing nothing is important unless it can be explained logically
• Being biased against illogical people

...and so on.

The key is to use Logic as the SUBJECT MATTER that is explored in terms of a MC's or IC's Fixed Attitude. So, those examples would be best expressed for a Main Character as:

• The Main Character believes logic is the only valuable way to settle problems
• The Main Character is fixated on one particular flaw in logic
• The Main Character believes nothing is important unless it can be explained logically
• The Main Character is biased against illogical people

A WORD ABOUT LOGIC:

Logic is not inherently problematic, even if it is the subject matter being explored. So you may explore Logic without choosing the structural element "Logic" as the problem in your story. The MC Problem could end up being:

• Pursue: The Pursuit of Logic creates trouble for the MC
• Feeling: Having a Feeling for Logic creates trouble for the MC
• Trust: Trusting in Logic creates trouble for the MC
• Faith: Having Faith in Logic creates trouble for the MC
• Order: The Ordered nature of Logic creates trouble for the MC
• Accurate: The Accuracy of Logic creates trouble for the MC
• Inertia: The Inertia of Logic creates trouble for the MC

Etc. EVERY element is open to you to be the MC Problem when your subject matter is Logic.

ADDENDUM:

To put it all together, examples of a CHANGE Main Character (Bob) in the Fixed Attitude domain might look like any one of these:

• Bob believes logic is the only valuable way to settle his problems (MC Domain of Fixed Attitude), even though the pursuit of logic creates trouble for him (MC Problem of Pursue), and he only finds personal relief when he changes to avoiding logic all together (MC Solution of Avoid/Prevent).
• Bob is fixated on one particular flaw in logic (MC Domain of Fixed Attitude), even though trusting in logic creates trouble for him (MC Problem of Trust), and he only finds personal relief when he changes to testing logic rigorously (MC Solution of Test).
• Bob believes nothing is important unless it can be explained logically (MC Domain of Fixed Attitude), even though having faith in logic creates trouble for him (MC Problem of Faith), and he only finds personal relief when he changes to disbelieving the need for logic all together (MC Solution of Disbelief).
• Bob is biased against illogical people (MC Domain of Fixed Attitude), even though the ordered nature of logic creates trouble for him (MC Problem of Order), and he only finds personal relief when he changes to embrace the randomness of chaos (MC Solution of Chaos).

Note that I chose to use the subject matter of Logic at both the throughline level and the problem level. They need not be the same, but it makes for a more consistent argument.